Due to increasing interest in protecting the environment in recent years, there has been a transition from cold-cathode fluorescent tubes, incandescent lamps, and the like to light sources that consume less power in display devices having a planar display or the like, lighting fixtures such as ceiling lights, laser devices, and illumination devices included in various types of optical systems such as sensors. Light sources using semiconductors, such as LEDs (light emitting diodes), have been given particular attention as a light source for illumination devices, due to their small size and low power consumption.
In an illumination device using LEDs as a light source, it is necessary to convert the wavelengths of primary light emitted from the LEDs to obtain a desired color tone. This conversion is performed via a wavelength conversion element. A phosphor layer that has phosphors therein is mainly used as this wavelength conversion element.
Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 2006-244779) discloses an illumination device using a wavelength conversion element. Furthermore, Patent Document 2 (Japanese Patent Application Laid-Open Publication No. 2001-318370) discloses a liquid crystal display device using a wavelength conversion element. Patent Document 3 (Japanese Patent Application Laid-Open Publication No. 2004-205953) discloses a wavelength conversion element itself.
The illumination device described in Patent Document 1 includes a light source unit having a plurality of light sources that emit primary light, and a wavelength conversion element that faces the plurality of light sources and that absorbs a portion of the primary light to emit secondary light of differing wavelengths. The wavelength conversion element includes a phosphor layer having phosphors therein and the phosphor layer is attached to the light source units such that at least a prescribed gap is maintained between the phosphor layer and the plurality of light source units.
In the liquid crystal display device described in Patent Document 2, the wavelength conversion element is formed on a primary surface of one of the pair of substrates sandwiching the liquid crystal layer. The wavelength conversion element includes a red phosphor layer, green phosphor layer, and blue phosphor layer. The respective phosphor layers have phosphors that are aligned with aligned liquid crystal polymers.
In this type of configuration, the liquid crystal polymers and phosphors included in the respective phosphors layers that constitute the wavelength conversion element are aligned, which allows for the degree of polarization of linearly polarized light emitted from the phosphors to be increased.
The wavelength conversion element described in Patent Document 3 includes a phosphor layer formed by dispersing birefringent anisotropic polymers and phosphors in a transmissive resin. The polymers have a different refractive index than the transmissive resin, which aligns the polymers in a prescribed direction in the phosphor layer.
In this type of configuration, aligning the birefringent anisotropic polymers in a prescribed direction in the phosphor layer causes the polymers to scatter secondary light emitted from the phosphors in order to efficiently extract linearly-polarized light having a prescribed plane of vibration.